In the oil and gas industry, seismic prospecting techniques are commonly used to aid in the search for and evaluation of subterranean hydrocarbon deposits. In seismic prospecting, an energy source is used to generate a seismic signal which propagates into the earth and is at least partially reflected by subsurface seismic reflectors (i.e., interfaces between underground formations having different elastic properties). The reflections are detected and measured by seismic receivers located at or near the surface of the earth, in an overlying body of water, or at known depths in boreholes, and the resulting seismic data may be processed to yield information relating to the subsurface formations.
The goal of all seismic data processing is to extract from the seismic data as much information as possible regarding the subsurface formations. Early seismic data processing efforts were directed primarily toward the development of methods for determining the geologic structure of the subsurface formations, and methods have been developed which permit geologic structure to be determined with a great deal of accuracy. More recently, however, substantial efforts have been directed toward the development of methods for using seismic data to determine the nature and geologic history of sedimentary rocks and their depositional environments. This is the field of seismic stratigraphy.
Seismic facies analysis is a subdivision of seismic stratigraphy used for regional stratigraphic interpretation. Seismic facies analysis examines the seismic character of a group of reflections to aid in defining the depositional environment, lithology, and geologic history of a seismic facies unit (i.e., a three-dimensional unit of seismic reflections whose characteristics differ from those of adjacent units). As used herein, "seismic character" means any recognizable aspect of a seismic event (e.g., a change in dominant frequency). In other words, the character of a seismic event is the waveshape which distinguishes it from other seismic events. Background information on seismic facies analysis may be found in Sheriff, R. E., 1980, Seismic Stratigraphy, published by International Human Resources Development Corporation, Boston, Mass., pp. 85-116.
Reflection character analysis is another subdivision of seismic stratigraphy which attempts to relate lateral trace-to-trace changes in waveshape to changes in the stratigraphy or interstitial pore fluid of a subsurface formation. These changes are typically in the amplitude, polarity, frequency, seismic velocity, or thickness (i.e., the interval between seismic events) of the reflected wave. Background information on reflection character analysis may be found in Sheriff, supra, pp. 161-183.
One property of seismic data which is useful in seismic stratigraphy is the continuity of the data. As used in connection with the present invention, "continuity" means the maximum lateral distance from a specified location and in a specified azimuthal direction for which the reflection character of a seismic event is essentially unchanged. Knowledge of the lateral continuity around any location is useful for a variety of purposes including, without limitation, checking the reliability of the related seismic interpretation, characterizing the subsurface stratigraphic interval in terms of its seismic facies, assisting in the placement of wells, and assessing the size and determining the volume of hydrocarbons-in-place for the region around an existing well or a proposed new well location.
Unfortunately, heretofore seismic reflection continuity has typically been estimated visually, either on standard displays of seismic data or on instantaneous phase displays. Although several methods have been proposed, there are currently no known methods for quantitatively measuring seismic reflection continuity for a stratigraphic interval which take into account the fact that both the shape and the amplitude of the signals must be considered in determining continuity, the local character of continuity (i.e., that continuity should be measured with respect to a specific location), or the angular (azimuthal) dependence of continuity.
One method which was previously proposed for quantitatively measuring seismic continuity is described in Lu, S. Y. and Cheng, Y. C., 1990, "An iterative approach to seismic skeletonization," Geophysics, vol. 55, pp. 1312-1320. This method attempted to develop pattern recognition algorithms for skeletonizing seismic reflection events. The skeletons of the seismic reflections were then used to measure the average skeletonized seismic reflection length for a window of traces along a 2D seismic line within a stratigraphic sequence. The resulting measure, however, was a window-averaged measure which necessarily depends on the choice of window width. This method does not consider the amplitude similarity of the data and does not recognize either the angular dependence of continuity or its local nature.
Another method which has been proposed for quantitatively measuring seismic continuity uses lateral variations in the spectral coefficient of coherence as a measure of continuity. See, Gibson, B. S. and Levander, A. R., 1990, "Apparent layering in common-midpoint stacked images of two-dimensionally heterogeneous targets," Geophysics, vol. 55, pp. 1466-1477 and Gibson, B. S., 1991, "Analysis of lateral coherency in wide-angle seismic images of heterogeneous targets," Journal of Geophysical Research, vol. 96, pp. 10261-10273. This method was originally developed for use with prestack seismic data, but could be generalized for use with stacked data. According to this method, the spectral coefficient of coherence, which is a function of frequency, is defined for a specified window. The coefficient is actually an average measure of the similarity of the seismic signals within the window and, therefore, necessarily depends on the choice of window width. Again, this method does not take into account the amplitude similarity of the data, the angular dependence of continuity, or the local nature of continuity. Moreover, the frequency dependence of coherence requires application of Fourier transformations to the seismic data, and because of the limitations of Fourier analysis for short time windows, this method is reliable only for seismic intervals much thicker than typically encountered.
The semblance coefficient has been proposed as a measure of lateral changes in seismic facies. See, Vossler, D. A., 1989, "Automatic delineation of lateral facies changes in clastic environments," Proceedings of the 59th Annual International Meeting, Society of Exploration Geophysicists, Paper SI 5.4, pp. 803-804, Tulsa, Okla. Although usually applied to prestack seismic data, this method could be generalized to stacked data to measure continuity. However, as with the coefficient of coherence, semblance is calculated for a specified window and, accordingly, necessarily depends on the choice of window width. Therefore, semblance does not measure a trace-to-trace geometric length having a specified azimuthal direction, but rather a property of the selected data window.
Finally, variogram analysis is another type of analysis which could be used to measure seismic continuity. The range of the variogram, i.e., the distance at which the spatial correlation vanishes along a given direction, is often mentioned as a measure of the "continuity distance" for the analyzed variable, and could be used as a spatial measure of continuity. However, the range of a variogram is an average value since it does not correspond to any specific location. Further, when seismic intervals of varying thicknesses are considered, as is often the case, the seismic signal for any given trace must be represented by a selected attribute. The variogram analysis is then applied to the selected attribute to estimate the continuity along a chosen direction. The resulting distance would be a reliable approximation of seismic continuity only to the degree that the selected attribute is a complete descriptor of the seismic character of the interval. If the selected attribute is not representative of the seismic character of the interval, then the result of the variogram analysis is not an accurate measure of seismic continuity.
From the foregoing discussion, it is apparent that there is a need for a reliable method for quantitatively estimating the lateral continuity of the seismic reflection character around any given location. The present invention satisfies this need.